This invention relates to a torque limiting coupling between a driving means, such as a power driven sprocket, and a driven means, such as a rotatable shaft keyed to one of a plurality of conveyor rollers in a continuously operative accumulating conveyor system. In such a system, a number of similar conveyor rollers rotatable about generally parallel side-by-side axes are provided for conveying articles or workpieces from one work station to another.
It is a commonplace to load the conveyor assembly with a number of the articles, which by way of example may comprise heavy automobile engine blocks, for conveyance to a work station where the articles or blocks are removed at intervals one at a time. In order to assure a continuous supply of engine blocks at the work station, a stop may be employed to engage the block and arrest the movement of the conveyor as each block in turn arrives at the work station. When each block at the work station is removed from the conveyor, the continuously operative conveyor moves the next successive block against the stop.
In a situation as described, it is desirable to provide continuous power to the driving means for the rollers, but to provide a torque limiting clutch or coupling between the driving means and each roller. Thus when a heavy engine block or other workpiece is waiting at the work station to be removed from the conveyor, the conveyor rollers underlying the block will cease rotation to prevent injury to the block or workpiece.
The prior art is replete with torque limiting couplings for use with roller conveyors as described, of which patents to Furlette et al, U.S. Pat. Nos. 4,056,953 and to Major, 4,143,525 are representative. Both show resiliently yieldable torque transmitting roller members between the cylindrical surface of a driving means and a driven shaft of polygonal cross section for imparting nominal driving torque to the shaft when the roller member engages a flat portion of the polygonal shaft and for enabling independent rotation of the driving means with respect to the driven shaft by resilient deformation of the roller member to override the apices of the polygonal shaft when the torque required to rotate the shaft exceeds a predetermined maximum.
A consequence of the prior art structures described is that a sharp pulsating torque is applied to the driven shaft when rotation of the conveyor roller keyed thereto is stopped, as for example when an engine block carried by the conveyor roller is stopped at the work station. Each time the resiliently yieldable torque transmitting member passes an apex portion of the driven shaft, the conveyor roller keyed to the shaft receives a momentary torque impulse, often causing a momentary rotation of the conveyor roller and damage to the overlying engine block of workpiece on the conveyor.
An important object of the present invention is to provide an improved, simple, and economical torque limiting coupling adapted for use with a continuously operative accumulating roller conveyor whereby pulsations in the driving torque applied to the conveyor roller are distributed over a comparatively large time interval or otherwise rendered sufficiently unobjectionable to avoid damage to parts carried by the conveyor roller when its rotation is stopped or impeded.
Other and more specific objects are to provide such a coupling comprising driving and driven means rotatable about a common axis, each means having a cylindrical surface, one within the other, dimensioned to provide a space between the cylindrical surfaces entirely around the axis of rotation. Although the cylindrical surfaces are generally circular in transverse section, they need not be true circles and may be slightly out-of-round cam surfaces to effect the desired shape of the aforesaid space as described herein. A torque transmitting member of resiliently yieldable elastomeric material is arranged within the space in frictional torque transmitting engagement with the two cylindrical surfaces to transmit driving torque to the cylindrical surface of the driven means from the cylindrical surface of the driving means upon rotation of the latter.
The torque transmitting member is preploaded to a resiliently deformed condition by means of an interference fit between the cylindrical surfaces, thereby to rotate the driven surface until its rotation requires a predetermined maximum allowable torque. When the torque required to rotate the driven means exceeds the maximum allowable torque, as for example when a conveyor roller rotated by the driven member is prevented from rotation by a heavy engine block thereon restrained against movement at a work station, the driving means will continue to rotate independently of the driven means. The latter will cease to rotate.
In one embodiment of the invention, the two cylindrical surfaces are arranged coaxially and are rotatable about their major common axis. As the torque required to rotate the driven means increases, relative movement between the torque transmitting member and at least one of the cylindrical surfaces may take place and gradually decelerate the rate of rotation of the driven means with respect to the driving means. In any event, at the maximum allowable torque, relative movement of at least one of the cylindrical surfaces with respect to the torque transmitting member will take place to enable continued rotation of the driving means independently of the driven means.
Although the undeformed shape of the torque transmitting member need not be cylindrical, and may be square in cross section or concavo-convex in order to conform to the cylindrical surfaces which it engages, a cylindrical roller type member rotatable about an axis parallel to the axis of rotation of the driving and driven members is preferred in order to minimize sliding contact and fractional wear. By virtue of the elastomeric resiliently yieldable torque transmitting roller and its interference fit between the cylindrical surfaces of the driving and driven means, the torque transmission may be primarily a function of the force required to deform the elastomeric roller. At the maximum allowable torque when rotation of the driven means ceases, the torque transmitting member or roller will move circumferentially within the annular space between the cylindrical surfaces of the driven and driving means upon continued rotation of the latter. In consequence, a constant maximum allowable torque will be applied to the driven means and torque pulsations which would otherwise be transmitted to the conveyor roller by the above-mentioned prior structures are entirely avoided.
In another embodiment of the invention, the major cylindrical axis of the driven means is arranged slightly eccentrically with respect to the common axis of rotation, i.e. the major axis of the cylindrical surface of the driving means. The space between the two cylindrical surfaces around the axis of rotation will thus have a radial dimension that decreases in both circumferential directions from a maximum radial dimension to a diametrically opposite minimum radial dimension. The operation of the resiliently yieldable torque transmitting member will be essentially the same as described above, except that as the torque required to rotate the driven means progressively increases to the maximum allowable torque, the torque transmitting member will be moved circumferentially from the maximum radial dimension and will be progressively compressed resiliently to increase the torque transmission gradually until the elastomeric member arrives at the location of minimum radial dimension to effect transmission of the maximum allowable torque.
If rotation of the conveyor roller is stopped, the increase in torque from the normal driving torque to the maximum allowable torque will be distributed over an appreciable time interval required for movement of the torque transmitting member from the region of the maximum radial dimension to the region of the minimum radial dimension. The application of sudden torque impulses to the conveyor roller and consequent momentary rotational movements or jerks thereof and damage to a workpiece carried thereon are thus avoided.
The above follows from the fact that the force required to deform a resiliently deformable member is inversely proportional to the time interval required for the deformation, neglecting friction. Other factors being equal in regard to the elasticity of the member, speed of rotation, etc., the slower and more gradual the torque build-up, the less will be the torque impulse transmitted to the conveyor roller.
In the Furlette et al construction, U.S. Pat. No. 4,056,953, where the apices of a hexagonal shaft are employed to deform the elastomeric torque transmitting roller, the attainment of maximum deformation is distributed over a 30.degree. arc of the roller movement about the axis of the cylindrical driving surface, i.e. the arc of roller movement from the mid-point of one of the flat surfaces of the hexagonal shaft to the maximum deformation effected by the next adjacent apex. At the least, the maximum torque impulse applied to the hexagonal shaft will be approximately six times the maximum toque impulse effected by the present invention where the deformation is distributed over 180.degree. of roller movement.
Actually the above analysis of the prior art understates the objections because for approximately half the 30.degree. arc of deformation movement, the deformable roller moves along a flat surface of the hexagonal shaft. The major rapid deformation takes place in the remaining approximately one-half of the 30.degree. arc of movement. The time duration for the maximuam deformation is thus appreciably less than the time required for the aforesaid 30.degree. arc of rotation.
In addition, the above-noted prior art structures effect a comparatively sharp penetration by the shaft apex into a localized region of the resiliently deformable roller at the time of maximum allowable torque, whereby the useful life of the roller material is rapidly terminated by fatigue. In contrast, the present invention compresses a roller gradually between two smooth cylindrical surfaces. The effect of both the rapidity of the deformation and extent of localized deformation are minimized and the operating life of the deformable roller is appreciably lengthened.
Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.